1. Field of the Invention
The present invention relates to a numerical controller for controlling a machine tool to machine a workpiece to a desired oval form, in accordance with a profiling program.
2. Description of the Prior Art
In a numerical controller for controlling the machining of an oval workpiece such as an automotive cam, generally data corresponding to the feed amount of a tool slide per unit angle rotations of the workpiece are programmed on a block-by-block basis, and as a series of profiling data blocks, are stored in a memory device prior to a machining operation. The profiling data blocks are successively read out from the memory device respectively when the unit angle rotations of the workpiece are effected, and an infeed amount through which the tool slide is to be infed in accordance with a feed sequence program during each unit angle rotation of the workpiece is added to a feed amount included in each of the read-out profiling data blocks. Each of the profiling data blocks, after such addition, is then subjected to a pulse interpolation processing so as to derive interpolation data, so that by effecting a pulse distribution operation in accordance with the interpolation data, there can be performed a numerical control to machine a workpiece to a desired oval form.
The reading-out of each of the profiling data blocks and the data processings for addition of the infeed amount to the read-out profiling data as well as for interpolation operation are performed in a parallel time relation, as shown in (a) and (b) of FIG. 4, with a pulse distribution operation executed in accordance with interpolation data which was calculated in a previous cycle. In a grinding process wherein the speed of pulse distribution is as fast as that in rough grinding, however, the reading-out of each profiling data block and the data processing are completed after the termination of a pulse distribution, as a result of the considerable time of duration thereof, and this makes it impossible for the prior art numerical controller to increase the speed of pulse distribution and to thereby enhance the machining efficiency in a rough grinding operation.
Particularly, in a so-called "computerized numerical controller" incorporating a general purpose digital computer of a small capacity type, a maximum speed of pulse distribution is constrained to a considerably low value due to the fact that data processing steps for addition of the infeed amount, interpolation operation and the like, take much time during execution by means of software, and that data processing steps for feed rate overriding control, speed-up and slow-down control, the calculation of a present position and the like must be executed each time the workpiece is rotated a unit rotational angle. By way of example, in the case where the data processing for a pulse distribution is executed upon each occurrence of a 0.1 degree rotation of the workpiece, so as to cause a grinding machine to perform a cam profiling motion, it was impossible to increase the rotational speed of the workpiece in rough grinding by more than 3 r.p.m.
Further, there may be considered another method, wherein a profiling program for rough grinding is prepared including a series of data blocks which are one-quarter or one-fifth of those of the above-noted profiling program for fine grinding and wherein a pulse distribution in rough grinding is executed in accordance with the profiling program for rough grinding. In this method, since the number of the read-outs and data processing steps which are executed concerning the profiling data during one rotation of the workpiece is decreased, it is possible to increase the pulse distribution speed in rough grinding and, accordingly, the workpiece rotational speed. However, the method not only necessitates providing in the memory device a data area for storing the profiling program for rough grinding, independently of that for storing the profiling program for fine grinding, but also necessitates preparing the profiling program for rough grinding, thus making the program preparation time consuming, complicated and troublesome.